Journal of Geosciences, 63 (2018), 265–276

DOI: 10.3190/jgeosci.267

Original paper

Horákite, a new hydrated bismuth uranyl–arsenate–phosphatemineral from Jáchymov (Czech Republic) with a uniqueuranyl-anion topology
Jakub Plášil1*, Anthony R. Kampf2, Jiří Sejkora3, Jiří Čejka3, Radek Škoda4, JaromírTvrdý4
Institute of Physics, Academy of Sciences of the Czech Republic (ASCR) v.v.i, Na Slovance 2, 182 21 Prague 8, Czech Republic;
plasil@fzu.cz
2
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA
3
Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, 193 00 Prague 9, Czech Republic
4
Department of Geological Sciences, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic*
Corresponding author
1

Horákite, ideally (Bi7O7OH)[(UO2)4(PO4)2(AsO4)2(OH)2]·3.5H2O, is a new uranyl mineral discovered on a specimen
originating from Jáchymov, Czech Republic (most probably from the Geister vein, Rovnost mine). It occurs as a supergene alteration mineral in association with phosphuranylite (overgrowing older metatorbernite–metazeunerite) in aquartz gangue with abundant tennantite. Horákite forms greenish-yellow to pale yellow prismatic crystals clustering toacicular aggregates, up to 1 mm across. Crystals are transparent to translucent with a vitreous luster. The mineral has alight yellow streak. Estimated Mohs’ hardness is ~2. The cleavage is perfect on {100}. The calculated density is 6.358
g/cm3. Horákite is optically biaxial (+), α ≈ 1.81, β ≈ 1.84, γ ≈ 1.88 (measured in white light); 2Vobs. is 78(1)°, 2Vcalc. is83°; non-pleochroic. The optical orientation is X = b, Z ≈ c. Electron-microprobe analysis yielded the empirical formula(Bi7.01Pb0.14)O7OH[(U1.01O2)4(P1.03O4)2(As0.74Si0.23O4)2(OH)2]·3.5H2O based on 37.5 O apfu. Horákite is monoclinic, C2/c,
a = 21.374(2), b = 15.451(3), c = 12.168(2) Å, β = 122.26(1)° and V = 3398.1(10) Å3, Z = 4. The eight strongest X-ray
powder-diffraction lines are [dobs Å(I)(hkl)]: 11.77(100)(110), 6.21(23)(–202), 5.55(23)(310, –112), 4.19(27)(–331),
3.54(61)( 510, –423), 3.29(20)( 331), 3.14(58)(241, 023) and 3.02(98)(150, 113, –533, mult.). The crystal structurerefinement of horákite, refined to R = 5.95 % for 1774 unique observed reflections, revealed a novel sheet structure. It
consists of topologically unique [(UO2)4(PO4)2(AsO4)2(OH)2] sheets (i.e., horákite topology), and an interstitial {(Bi7O7OH)(H2O)3.5} complex. Sheets result from the polymerization of UO7 bipyramids by sharing edges to form tetrameric units;
tetrahedrally coordinated sites are linked to the UO7 both monodentately (T1 to U1) and bidentately (T2 to U2). The
mineral is named after František Horák (1882–1919), the mining engineer in Jáchymov, and his grandson, Vladimír
Horák (born 1964), an amateur mineralogist and expert on the mining history of the Jáchymov ore district.
Keywords:horákite, new mineral, uranyl arsenate, crystal structure, topology, Jáchymov
Received: 7 February 2018; accepted: 20 August 2018; handling editor: F. Laufek
The online version of this article (doi: 10.3190/jgeosci. 267) contains supplementary electronic material.

1. Introduction
Uranyl phosphate and arsenate minerals are environmentally important phases that result from hydration–oxidation
weathering of primary U minerals, mainly uraninite (Finchand Murakami 1999; Krivovichev and Plášil 2013; Plášil
2014). Generally, due to their low solubility products (seee.g., Ilton et al. 2010; Astilleros et al. 2013; Göb et al.
2013), they can occur both in the very leached parts of theuranium deposits (Finch and Murakami 1999; Plášil et al.
2006, 2009; Göb et al. 2013) and in mine dumps, wastes
and tailings (Buck et al. 1995; Roh et al. 2000; Catalano
et al. 2006; Cantrell et al. 2011; Maher et al. 2013). This
makes uranyl phosphate and arsenate minerals essential
in controlling U mobility in the environment. Currently,

there are approximately sixty valid uranyl phosphate andarsenate species. Herein, we report on the new mineralhorákite, ideally (Bi7O7OH)[(UO2)4(PO4)2(AsO4)2(OH)2]·
3.5H2O, which is the first uranyl mineral containing both
phosphate and arsenate as essential components.
The new mineral honors two remarkable persons: mining engineer František Horák (1882–1919), who served
from 1916 to 1918 as a formal chief of the Radium factoryin St. Joachimsthal (Jáchymov), and his grandson, MUDr.
(M.D.) Vladimír Horák (born 1964), who is an amateur
mineral collector and historian, focused on the history ofmining in Jáchymov. The new mineral was approved by
the IMA Commission on New Minerals, Nomenclature andClassification (IMA 2017-033). The holotype specimen canbe found in the mineralogical collection of the Nationalwww.jgeosci.org

Jakub Plášil, Anthony R. Kampf, Jiří Sejkora, Jiří Čejka, Radek Škoda, Jaromír Tvrdý

Museum in Prague (Czech Republic) under the catalog
number P1P 17/2017. The crystals from the holotype usedin this study are deposited in Natural History Museum of
Los Angeles County (USA) under catalog number 66575.

2. Occurrence
Horákite was found on an old sample (probably discovered during first half of the 19th century) originating from
Jáchymov (St. Joachimsthal), Czech Republic, preservedin a private collection of one of the authors (JT), and is
now stored as a holotype. Despite the absence of the exact
location, we can infer that sample comes from the Rovnost(formerly Werner) mine. Thorough details on the mineralogy, geology, and history of the Jáchymov ore district canbe found elsewhere (Ondruš et al. 2003; Hloušek et al.
2014). The matrix of the fist-sized holotype sample is a (mylonitized) mica-schist containing thin quartz veinlets. The
surface of the specimen exhibits abundant phosphuranylite
overgrowing older metatorbernite–metazeunerite. Horákite
occurs very sparingly in vugs of the quartz gangue. Primaryminerals are represented by relics of tennantite and finegrained uraninite. This association is typical of the Geistervein in the Rovnost mine (see, e.g. Plášil et al. 2017). The
new mineral is of supergene origin, having formed in-situ
in the oxidation zone. The inferred paragenetic sequence ismetatorbernite/metazeunerite → phosphuranylite + horákite.

3. Physical and optical properties
Horákite occurs as prismatic to bladed crystals forming aggregates up to 1 mm across (Fig. 1). Crystals are

Tab. 1 Chemical composition of horákite from Jáchymov
Constituent
PbO
Bi2O3
UO3
SiO2
P 2O 5As2O5
H2O*

Mean of 21 spots(wt. %)
0.99
50.22
35.58
0.85
4.47
5.21
2.77

Range

SD

Standard

0.00–1.75
49.00–51.33
33.47–37.66
0.60–1.27
4.09–5.91
4.28–5.91

0.04
0.63
0.91
0.13
0.41
0.41

galenametallic Bi
uranophanesanidine
fluorapatitelammerite

Total
100.09* H2O content was calculated from stoichiometry (5 H2O) derived fromthe crystal-structure study
SD – standard deviation

elongated on [001]. Crystals are transparent to translucent, greenish yellow to pale yellow and have a vitreous
luster. The mineral has a light yellow streak and a Mohs’
hardness of ~2. Prismatic crystals show at least one
cleavage, perfect on {100}. A density of 6.358 g/cm 3
was calculated using the empirical formula and unit-cellparameters obtained from single-crystal X-ray diffraction.
Horákite is non-fluorescent under short- or long-wave
UV radiation. The small size of the crystals and theirhigh indices of refraction made the determination of theindices of refraction very difficult and only approximate
values can be provided. Horákite is optically biaxial (+),
with α ≈ 1.81, β ≈ 1.84, γ ≈ 1.88 (measured in whitelight). The 2V is 78(1)°, determined from extinction datausing EXCALIBR (Gunter et al. 2004); the calculated
2V is 83°. Dispersion could not be observed. No pleo­chroism was observed. The optical orientation is X = b,Z ≈ c. The Gladstone-Dale compatibility, 1 – (Kp/Kc),
is –0.0568 (good) using the empirical formula, wherek(UO3) = 0.118, as provided by Mandarino (1976). As
noted above, the measured indices of refraction are, at best,
approximations.

4. Chemical   composition
The chemical composition ofhorákite was determined usinga Cameca SX100 electron microprobe (WDS mode, 15 kV, 2
nA, and 5 µm beam diameter)
at Masaryk University in Brno.
Peak counting times were 10–20
Fig 1 Horákite crystal aggregate in aquartz gangue vug in association withblackish aggregates of tennantite (holotype specimen). Horizontal width ofthe picture is 2 mm, photo P. Škácha.

266

Horákite, a new uranyl mineral from Jáchymov

multiple polystyrene Raman bands (laser frequency calibration) and standardized white-light sources (intensity
calibration). Spectral manipulations were performed using the Omnic 9 software (Thermo Scientific). Becauseof the complex structure of horákite (including P/As
substitution and symmetrically non-equivalent U atoms),
the coincidences and overlaps make any tentative attribu-

Raman intensitys and the counting time for the background their half.
The measured intensities were processed for matrix effects using the “PAP” correction routine (Pouchou andPichoir 1985). The H2O content (5 H2O in total) was
calculated by stoichiometry (obtained from the structure)on the basis of 37.5 O apfu. The analytical results arereported in Tab. 1. No other elements were detected; inparticular, Al, Ca, Cl, F, Fe, K,
Mg, Mn, Na and V, were soughtaand found to be below detection
limits (~0.03–0.05 wt. % at theanalytical conditions used).
The empirical formula ofhorákite (calculated on the basisof 37.5 O apfu) is (Bi7.01Pb0.14)
O7OH[(U1.01O2)4(P1.03O4)2(As0.74
Si 0.23O 4) 2(OH) 2]·3.5H 2O; thisformula is well-balanced, showing 0.01 excess of a negative
charge only. The ideal formulaof horákite is (Bi7O7OH)[(UO2)4(PO 4) 2(AsO 4) 2(OH) 2]·3.5H 2O,
which requires Bi 2 O 3 50.38,
UO 3 35.35, P 2O 5 4.39, As 2O 5
7.10 and H 2O 2.78, total 100
wt. %.

The Raman spectrum was collected in the range 4000–30
cm –1 using a DXR dispersive
Raman Spectrometer (Thermo
Scientific) mounted on a confocal Olympus microscope. The
Raman signal was excited bya green 532 nm diode-pumped
solid-state laser and detectedby a CCD detector. The experimental parameters were: 10×
objective, 10 s exposure time,
100 exposures, 400 lines/mm
grating, 25 μm pinhole spectrograph aperture and 0.4 mW
laser power level. The instrument was set up by a softwarecontrolled calibration procedureusing multiple neon emission
lines (wavelength calibration),Fig. 2a – Raman spectrum of horákite
in the range 4000–30 cm–1 (split at 2000cm–1). b – Raman spectrum of horákite
in the range 1800–30 cm–1.

4000

3000

2000

1500

500

1000

Raman shift (cm )–1bRaman intensity

5. Raman spectroscopy

1800

1600

1400

1200

1000

800

600

400

200

Raman shift (cm–1)

267

Jakub Plášil, Anthony R. Kampf, Jiří Sejkora, Jiří Čejka, Radek Škoda, Jaromír Tvrdý

vibrations and splitting of doubly- or triply-degenerate
ones. The Raman spectrum was interpreted with reference
to Nakamoto (1986), Čejka (1999), Sejkora et al. (2002,
Icalc.
dcalc.
h
k
l
Irel.
dobs
2004, 2008), Ardelean et al. (2006) and Frost et al. (2011)100
11.745
1
1
0
100
11.77
and references therein.
11
6.079
–2
0
2
23
6.21
13
5.613
3
1
0
The full-range Raman spectrum of horákite is shown
23
5.55
12
5.460
–1
1
2
in Fig. 2a. An observed weak broad-band at 3580 cm–1
13
4.949
–4
0
2
13
4.99
with a shoulder at 3410 cm–1 is connected with the ν O–H
19
4.171
–3
3
1
27
4.19
stretching vibrations of hydrogen-bonded OH– and H2O
13
3.863
0
4
0
15
3.87
molecules. Approximate O–H…O hydrogen bond lengths
11
3.702
2
0
2
vary in the range from 3.2 to 2.81 Å (Libowitzky 1999).
17
3.69
10
3.616
0
4
1
The spectrum in the range 1800–30 cm–1 is shown
26
3.520
5
1
0
in Fig. 2b. No Raman band was observed in the region
61
3.54
17
3.495
–4
2
3
where the ν2 (δ) H–O–H bending vibrations should oc12
3.298
3
3
1
20
3.29
cur. A series of weak bands between 1200 and 1030 cm–1
10
3.159
2
4
1
(1103, 1081, 1069, 1055 and 1039 cm–1) may be attrib58
3.14
12
3.145
0
2
3
uted to the triply degenerate ν3 antisymmetric stretching
16
3.046
1
5
0
vibration of (PO4)3– polyhedra; those between 1030 and
13
3.022
1
1
3
98
3.02
930 cm –1 to the ν 1 symmetric stretching vibration of
28
2.973
–5
3
3
(PO4)3– polyhedra.
14
2.918
–2
0
4
35
2.92
The ν 3 (UO 2) 2+ vibrations, activated in the Ramand values quoted in Å
spectrum by symmetry distortion, may be connectedwith the bands at 879 and 864 cm–1, while the ν1 (UO2)2+
tion of observed bands to individual vibrations difficult.
symmetric stretching vibration is present as a very
Observed lowering of symmetry of the structural units
strong band centered at 801 cm–1 with a shoulder at 850
causes corresponding Raman activation of some or all
cm–1. Bartlett and Cooney (1989)
provided an empirical formula toTab. 3 Crystallographic data and refinement details for horákite
derive the approximate U–O bond
Structural formula(Bi7O7OH)[(UO2)4(PO4)2.741(AsO4)1.259(OH)2]·3.5H2O
lengths from the band positions
a, b, c [Å]
21.374(2), 15.451(3), 12.168(2)assigned to the (UO2)2+ stretchingβ [°]122.261(13)vibrations (Å/cm –1 ): 1.80/879,
3398.1(10)V [Å3]
1.81/866, 1.76/850, 1.81/801.
Z
4
These values are in accordance
6.263
Dcalc [g/cm3]
with U–O bond lengths given inSpace groupC2/m
general by Burns et al. (1997) and
Temperature
297 K
Lussier et al. (2016) for the U–O
Diffractometer; wavelength
Rigaku SuperNova with Atlas S2 CCD detectorbond-length in UO 7 polyhedra
X-ray radiation/power
MoKα (0.71073 Å)/50 kV, 40 mA
and from the current crystal-strucCrystal dimensions [mm]0.020 × 0.012 × 0.010
ture data. A coincidence of the ν1
Collection mode
ω rotational scans(UO 2) 2+ stretching, the δ–UOH
Limiting θ angles [°]3.70–26.81°(in-plane) bending modes and theLimiting Miller indices–22<h<26, –15<k<21, –14<l<16
No. of reflections
8703
triply degenerate ν3 antisymmetricNo. of unique reflections
3574
stretching vibration of (AsO4) tetNo. of observed reflections (criterion)
1744 [Iobs>3σ(I)]rahedra in this region is assumed.
0.128
Rint
The band at 774 cm –1 is asAbsorption correction (mm–1), method
56.56, Gaussian (Jana2006)
signed to the ν1 symmetric stretchTransmission (min/max)
0.4429/0.6699
ing vibration of (AsO4). A series
5325
F000
of weak bands observed in the
Refinement by Jana2006 on F2
640–520 cm–1 region may be atParameters refined, constraints, restraints
152, 0, 21
tributed to the triply degenerate
R, wR (obs)0.0595, 0.1131
ν 4 bending vibrations of (PO 4)
R, wR (all)0.1350, 0.1458
tetrahedra
and Bi–O stretching viGOF obs/all
1.22/1.09
brations.
The
complex composite–3–6.08, 11.51 (0.11 Å to Bi3)
Δρmin, Δρmax (eÅ )band
between510 and 360 cm–1 is2
2
Weighting scheme, details
σ, w = 1/(σ (I) + 0.0004I )Tab. 2 Powder X-ray diffraction data for horákite (only calculated lines
with Irel. > 1 are listed)268

Horákite, a new uranyl mineral from Jáchymovprobably connected with the doubly
degenerate ν 2 bending vibrationsof (PO 4), the triply degenerate ν 3
bending vibrations of (AsO4), Bi–O
stretching and Bi–O–Bi bending vibrations. The bands at between 380
and 280 cm–1 may be assigned to the
doubly degenerate ν2 bending vibrations of (AsO4) and Bi–O stretchingvibrations. According to Ardelean
et al. (2006), bands close to ~235
cm–1 may be assigned to the Bi–O
bonds vibrations in BiO3 and BiO6
polyhedra, bands close to ~315 cm–1
to Bi–O–Bi stretching vibrations in
distorted BiO6 polyhedra, and bands
close to ~585 cm –1 to the Bi–O
stretching vibrations in BiO6 polyhedra. The band at 251 cm–1 withshoulders at 271 and 228 cm –1 isconnected with the doubly degenerate ν2 bending vibrations of (UO2)2+groups. The remaining bands < 200
cm–1 (189, 163, 147, 123, 105, 74
and 48 cm–1) are assigned to externallattice vibration modes and (UO2)2+translations and rotations.

6. X-ray crystallography
and crystal structure

Tab. 4 Atom positions and displacement parameters (in Å2) in the structure of horákite
Atom

Occupancy

x/a

y/b

z/c

Bi1
0.55568(7)0.59587(8)0.52965(11)Bi2
0.5
0.77560(12)0.25U10.80049(7)0.57582(8)0.64664(11)Bi30.37269(7)0.60815(9)0.23420(12)Bi40.41117(7)0.39403(9)0.11847(12)U20.69234(8)0.82367(9)0.83708(13)As1/P10.57(2)/0.43(2)0.6842(2)0.7689(3)0.5517(4)P2/As20.94(2)/0.06(2)0.7521(4)0.5905(5)0.8885(7)O10.4750(11)0.6828(13)0.3449(18)O2
0.6774(12)0.5491(15)0.803(2)O30.4392(11)0.5459(13)0.4273(18)O40.4726(11)0.3751(13)0.3278(18)O50.7813(14)0.8500(17)0.857(2)O6
0.7620(13)0.7165(15)0.621(2)O7
0.7970(14)0.5993(17)0.826(2)O8
0.6060(15)0.7903(18)0.824(2)O90.7389(12)0.6873(15)0.924(2)O10
0.6216(12)0.7231(14)0.4113(19)O110.6511(12)0.7619(14)0.6426(19)O120.8938(12)0.6088(14)0.733(2)O13
0.7031(13)0.9524(16)0.976(2)O14
0.4327(11)0.5307(13)0.1590(18)O150.7048(12)0.5478(14)0.5613(19)O160.6985(13)0.8696(15)0.533(2)O17
0.6489(15)0.9442(19)0.725(2)O18
0.5
0.909(3)0.729(4)0.975(5)O19
0.5
0.528(4)0.868(5)0.463(6)O20
0.5
0.596(4)0.973(4)0.508(6)O21
0.25
0.503(7)0.931(8)0.687(10)
Ueq is defined as a third of the trace of the orthogonalized Uij tensor* – refined with isotropic displacement parameters

The X-ray powder-diffraction datafor horákite (Tab. 2) were recorded using a Rigaku
R-Axis Rapid II curved imaging plate microdiffractometer with monochromated MoKα radiation. A Gandolfilike motion on the φ and ω axes was used to randomize
the sample. Theoretical diffraction powder pattern was
calculated by PowderCell program (Kraus and Nolze
1996) from crystal-structure data. Due to a low qualityof the powder-diffraction pattern and a low symmetry of
horákite structure only comparison between calculated
and observed d-spacing and intensities is given.
A 0.020 × 0.012 × 0.010 mm prismatic crystal ofhorákite was selected for single-crystal X-ray diffraction
experiment using a Rigaku SuperNova single-crystal fourcircle diffractometer with kappa-geometry and mirrormonochromatized MoKα radiation (λ = 0.71073 Å) from a
micro-focus X-ray tube. Diffracted X-rays were recordedwith an Atlas S2 CCD detector. Horákite is monoclinic,
with space group C2/c, and cell parameters: a = 21.374(2),b = 15.451(3), c = 12.168(2) Å, β = 122.26(1)° andV = 3398.1(10) Å3, Z = 4. The summary of experimental
conditions, crystallographic data and refinement results
is listed in Tab. 3.

Ueq
0.0138(5)0.0143(8)0.0135(5)0.0183(6)0.0196(6)0.0225(6)0.011(2)0.013(4)0.011(2)*0.023(6)*0.011(2)*0.011(2)*0.034(7)*0.020(6)*0.031(6)*0.041(8)*0.020(6)*0.014(5)*0.016(5)*0.019(5)*0.023(6)*0.011(2)*0.017(5)*0.021(6)*0.043(8)*0.044(15)*0.08(2)*0.059(19)*0.05(3)*The structure of horákite was solved from the singlecrystal X-ray data using the SHELXT program (Sheldrick2015) and refined by the full-matrix least-squares algorithmof the Jana2006 software (Petříček et al. 2014) based on F2.
The initial structure solution suggested unambiguously aC-centered cell and the centrosymmetric space groupC2/c, which was confirmed by the subsequent refinement.
Several atoms missing in the initial structure model (e.g.,
O18, O19, O20, O21) were found from the difference
Fourier maps. Two tetrahedrally coordinated sites wererefined with joint occupancies by P and As. Only for U,
Bi and P/As sites the anisotropic atomic displacement
parameters were used; in case of O atoms isotropic displacement parameters were used instead. A final difference
Fourier map failed to indicate possible locations of H
atoms, which is not unusual for a U compound; moreover, here it is due to weak data. The refinement for 151
parameters converged smoothly to a final R = 0.0573, wR= 0.1017 for 1743 observed reflections with goodness offit (GOF) of 1.16. Refinement results are given in Tab. 3,
atomic coordinates and displacement parameters in Tab. 4,
anisotropic displacement parameters in the Electronic
269

Jakub Plášil, Anthony R. Kampf, Jiří Sejkora, Jiří Čejka, Radek Škoda, Jaromír Tvrdý

ear uranyl ion – UO22+ (Tab. 5).
In the equatorial plane, each uraU1–O6
2.29(2)
U2–O5
1.83(3)nyl ion is linked to five ligands,
U1–O7
2.25(3)
U2–O8
1.84(3)either O or OH (O17 atom).
U1–O12
1.76(2)
U2–O9
2.33(2)Uranyl polyhedra polymerize by
U1–O13v
2.39(3)U2–O9vii
2.46(2)sharing equatorial edges to form
U1–O15
1.78(2)
U2–O112.25(2)
tetrameric units; tetrahedrally
U1–O16iii
2.35(3)U2–O13
2.54(3)U1–O17v
2.44(3)U2–O172.20(3)
coordinated sites are linked to<U1–OUr>
1.77<U2–OUr>
1.83
the UO7 bipyramids in two dis<U1–Oeq>
2.34<U2–Oeq>
2.36
tinct ways, as monodentate (T1
to U1) or bidentate (T2 to U2)
T1–O6
1.62(2)
T2–O2
1.51(2)linkages (Fig. 4). The T1 site
T1–O101.664(18)
T2–O7
1.52(4)was found to be dominantly ocT1–O11
1.60(3)T2–O9
1.62(2)
cupied by As5+. The T2 site is1.55(2)T1–O16
1.63(2)T2–O13vii
nearly fully occupied by P5+ (94< T1–O>
1.63< T2–O>
1.55%). Through T1 tetrahedra, adjacent four-membered clusters are
Bi1–O12.385(18)
Bi2–O12.08(2)
Bi3–O12.186(19)linked along [010]. These chainsii
i
2.08(2)
Bi3–O22.60(2)
Bi1–O23.016(19)Bi2–O1
are then linked via T–O bonds
Bi1–O32.24(2)
Bi2–O102.413(19)
Bi3–O3
2.213(18)to adjacent chains to form coriii
Bi1–O32.24(2)
Bi2–O102.413(19)
Bi3–O53.10(4)i
iii
ii
rugated sheets parallel to {100}Bi1–O42.17(3)
Bi2–O122.81(3)
Bi3–O102.56(2)(Fig. 5). Between these sheets,
Bi1–O14ii
2.64(3)
Bi2–O12iv
2.81(3)
Bi3–O14
2.27(3)
there is a (Bi7O7OH)6– complex,
Bi1–O15
3.09(3)
Bi2–O19
2.74(9)<Bi3–O>
2.49
ii
which involves one [6]-coordi<Bi1–O>
2.54
Bi2–O19
2.74(9)<Bi2–O>
2.51
nated, one [7]-coordinated andtwo [8]-coordinated Bi3+ cationsi
2.69(3)
Bi4–O2(Tab. 5) and H2O groups linked
Bi4–O3vi
2.85(3)by H-bonds to the sheets, as well
Bi4–O42.174(19)as to the Bi-complex. The Bi–O
Bi4–O4ii
2.22(2)bonds link Bi atoms into a 3D
Bi4–O52.97(2)
chain structure, which extends
Bi4–O83.00(3)parallel to c (Fig. 3). There are a
Bi4–O14
2.16(2)
few O atoms within the interstiBi4–O18
3.07(6)
tial complex (bonded to Bi) that
<Bi4–O>
2.64
have a low Ueq value. The low
Symmetry codes: (i) –x+1, –y+1, –z+1; (ii) –x+1, y, –z+1/2; (iii) –x+3/2, –y+3/2, –z+1; (iv) x–1/2,
Ueither suggests a possibleeq–y+3/2, z–1/2; (v) –x+3/2, y–1/2, –z+3/2; (vi) x, –y+1, z–1/2; (vii) –x+3/2, –y+3/2, –z+2; (viii) x+1/2,
presence of a heavier anion at–y+3/2, z+1/2; (ix) –x+3/2, y+1/2, –z+3/2; (x) x, –y+2, z+1/2; (xi) –x+1, –y+2, –z+1
the sites, maybe F – (however,no F was indicated in the WDS analysis), or it is just an
Supplementary Material (Tab. S1), selected interatomic
artifact due to poorly fitted absorption effects.
distances in Tab. 5, and results of bond-valence analysis
Therefore, we assume from the refinement the strucin Tab. 6. The bond-valence analysis was carried out foltural formula for horákite to be (Bi7O7OH)[(UO2)4(PO4)2.7
lowing the procedure of Brown (2002). The CIF file, also
(AsO4)1.259(OH)2]·3.5H2O, Z = 4, which is electroneutral.
containing a block with the reflections, is deposited at the41
However, the exact mechanism of the charge balance
Journal’s web page www.jgeosci.org.
in the interlayer remains unclear because of the limited
quality of the X-ray data.
Tab. 5 Selected interatomic distances (in Å) in the crystal structure of horákite

6.1. Crystal structure

The asymmetric unit of horákite (Fig. 3) contains two
U sites, four Bi sites, two T sites jointly occupied by P
and As, and twenty O sites (of which three belong to OH
and four to H2O; Tab. 6). Each U site is coordinated byseven ligands, forming pentagonal bipyramids, whereby
the apices of each bipyramid are comprised of strongly
bonded (~1.8 Å) O atoms, forming the approximately lin270

6.2. Structural and chemical complexity
The structural complexity of horákite and related minerals was determined as the Shannon information content
per atom (IG) and per unit cell (IG,total). The concept of
Shannon information, also known as Shannon entropy,
used herein originates from information theory. This

Horákite, a new uranyl mineral from Jáchymov

Fig. 3 Polyhedral representation of thehorákite structure viewed down to caxis (UO7 = yellow, TO4 = violet, Bi= blue, interstitial O = red). Unit-celledges are outlined by black solid lines.

Fig. 4 Fundamental building block of the uranyl phosphate sheet found in horákite: a tetramer of the edge-sharing UO 7 bipyramids (yellow) withtwo bidentate symmetrically related T2 tetrahedra and four symmetrically related T1 tetrahedra that link adjacent tetramers (ball-and-stick model)
into infinite chains along b. The O17 site (OH) is shown as a blue sphere.

approach was developed by Krivovichev (2012, 2013,2014, 2016, 2017): the complexity of a crystal structure
can be quantitatively characterized by the amount of

Shannon information, which is measured in bits (binary digits) per atom (bits/atom) and per unit cell (bits/cell), respectively. The amount of Shannon information
271

Jakub Plášil, Anthony R. Kampf, Jiří Sejkora, Jiří Čejka, Radek Škoda, Jaromír Tvrdý

Tab. 6 Bond-valence analysis of the horákite structure
U1
O1
O2
O3
O4
O5
O6
O7
O8
O9
O10
O11
O12
O13
O14
O15
O16
O17
O18
O19
O20
O21

U2

Bi1
Bi2
0.44 0.97×2↓
0.09
0.65×2↓
0.77

Bi3
0.74
0.25
0.69

7. Discussion
Bi4

0.20
0.13
0.76;0.68
0.10

T1*

T2
1.33

∑BV Assignment
2.15
O
1.87
O+H
1.47
OH*
1.49
OH*
1.69
O+H
1.91
O
1.97
O
1.59
O+H
2.03
O
1.85
O+H
2.04
O
1.90
O
2.09
O
1.61
O+H
1.75
O+H
1.81
O+H
1.20
OH
0.08
H 2O
0.36
H 2O
0.00
H 2O
0.00
H 2O

7.1. The topology of the   uranyl-anion sheets

Horákite, due to above-mentioned substitution, is the
0.62
1.30
P-dominated member of the
0.67
1.30
assumed series, containing,
1.49
0.09
however, significant amount
0.57;0.44
1.01
of As. We can only specu0.41×2↓ 0.28
1.15late about an ideal composi0.67
1.37
tion of the P-dominated end1.75
0.15×2↓
member sheet, which can be
0.51
0.38
1.20
[(UO 2 ) 4 (PO 4 ) 4 (OH) 2 ] 6– , and
0.23
0.59
0.79
of the As-dominated end1.68
0.07
member sheet that can be
0.55
1.26
[(UO 2) 4(AsO 4) 4(OH) 2] 6–. It is
0.46
0.74
0.08
a topologically unique sheet
0.36×2↓among known uranyl minerals
and synthetic compounds. The
graphical representation of the
∑BV
6.24
5.82
2.89
3.42
2.63
2.83
5.07
4.84
sheet anion topology, which
Values are given in valence units (vu)consists of pentagons, squares,
ΣBV = sum of bond-valences incident at the atomic site; OH* – the site has partially OH character;and triangles, is displayed in
O+H – there is a least one H-bond (~0.2 vu) accepted by the anion
6+
The bond valence parameters for U –O taken from Burns et al. (1997), other bond-valence parameters Fig. 6. Both tetrahedra are connected to three U polyhedra
from Gagné and Hawthorne (2015)
T1* – site considered as mixed site with occupancies 0.57 As, and 0.43 P
with the fourth vertex of eachtetrahedron non-linked withinreflects the diversity and relative proportion of differthe sheet (Figs 4 and 5). Thus, orientational stereoisoment objects, e.g., the number and relative proportion oferism can occur (Krivovichev 2009, 2010). In the sheet
different sites in an elementary unit cell of a crystal
anion topology of horákite, we can discern tetrahedra
structure.
(T1 and T2, respectively) related to empty squares and
The information-based structural-complexity values
tetrahedra related to distorted pentagons (i.e., bidentately
were calculated using the software package TOPOS (Blalinked tetrahedra) (Fig. 5). The empty squares between T1
tov et al. 2014). The chemical complexity (after Siidra etand T2 positions connect one down, and one up tetraheal. 2014) was estimated by considering chemical formuladron. There are also empty diamonds/rhomboids connectas a message, where symbols correspond to different
ing two T1 sites, one oriented up and one oriented down.
chemical elements. Structural and chemical complexity
There are also distorted pentagons that connect one up,
values are given in Tab. 7.
and one down, T1 and T2 tetrahedra (Fig. 5).
1.52

0.07

Tab. 7 The complexity measures for horákite and related minerals (calculations include H atoms)
MineralChemical formula

Spgr. V (Å3)

HügelitePb2[(UO2)3O2(AsO4)2](H2O)5
P21/m
Cmcm
Phosphuranylite K2Ca[(UO2)7(PO4)4O2(OH)2](H2O)8*
Horákite(Bi7O7OH)[(UO2)4(PO4)2.74(AsO4)1.26(OH)2]·3.5H2O C2/c
Kamitugaite
PbAl[(UO2)5(PO4)2.38(AsO4)0.62O2(OH)2](H2O)11.5
P–1
DumontitePb2[(UO2)3O2(PO4)2](H2O)5
P21/m
Hallimondite
Pb2[(UO2)(AsO4)2](H2O)0.5
P–1
P–1
Walpurgite(BiO)4(UO2)(AsO4)2(H2O)2

3762
3765
3398
1553
903
499
374

v
304
152
134
96
76
32
25

IG
(bits/atom)
6.41
5.04
5.11
5.59
4.41
4.00
3.68

IG, total(bits/cell)
1947.37765.69684.86536.16334.84128.0092.10

Ichem(bits/formula)
65.21
128.31111.91131.58
65.21
27.11
54.87

Reference
1)
2)
3)
4)
5)
6)
7)

Spgr. – space group; * – the electroneutral formula proposed for phosphuranylite, derived from new structure determinations (unpublished dataof authors). References: 1) Locock and Burns (2003), 2) Demartin et al. (1991), 3) this paper, 4) Plášil (2017), 5) Piret and Piret-Meunier (1988),
6) Locock et al. (2005), 7) Mereiter (1982)

272

Horákite, a new uranyl mineral from Jáchymov

U1

U2

T2

T1

Fig. 5 Uranyl phosphate sheet
found in the structure of horákite;
the O17 site (OH) is shown as a
blue sphere.

These empty diamonds/rhomboids are a rather
unique topological feature in the crystal chemistry of
U 6+ compounds. Somewhat similar are distorted diamonds in sheets of the synthetic phases K[(UO2)(CrO4)(OH)]·1.5H2O (Serezhkina et al. 1990), Cs[(UO2)(OH)(SeO 4 )]·H 2 O (Serezhkina et al. 2010) or Cs[(UO 2 )F(PO3OH)]·0.5H2O (Ok et al. 2006).

where, however, still some As5+ is present (from oxidationof tennantite). Somewhat similar circumstances were observed in case of the uranyl phosphate–arsenate mineralkamitugaite (Plášil 2017).

7.2. The presence of As5+ in the structure ofhorákite

As stated above, horákite represents a possible orderedintermediate member between two yet unknown endmembers. Their respective theoretical formulae are(Bi7O7OH)[(UO2)4(PO4)4(OH)2]·3.5H2O and (Bi7O7OH)[(UO2)4(AsO4)4(OH)2]·3.5H2O. It should be mentionedthat the U : T (T = As and/or P) ratio (4 : 4 or their factors)of the uranyl-anion sheet in horákite is similar to that in
autunite-group minerals (1 : 1; Locock 2007). Nevertheless, the metal(Me) : U : T is distinctive; for horákite it is
7 : 4 : 4 and for autunite-group minerals, e.g. for arsenuranospathite (Dal Bo et al. 2015), it is 1 : 2 : 2.
Horákite is a new member of Strunz class 08.ED;
unclassified uranyl phosphates and arsenates. Visually,
it most resembles parsonsite, hügelite and walpurgite;
however; these three minerals have completely distinct

It is highly interesting that in the horákite structure oneof the tetrahedral sites (As1/P1) was found to be slightly
dominated by As5+ over P5+, while the second site (P2/
As2) far more by P5+. It seems to be reasonable that larger
As5+ (ionic radius of 0.34 Å; Shannon 1976) is entering
monodentately linked T1 tetrahedron, while bidentately
linked T2 tetrahedron is occupied by much smaller P5+(0.17 Å; Shannon 1976). The repulsion in case of As5+
would probably destabilize the structure. We can only
speculate whether the As is essential for the stabilityof horákite. Nevertheless, horákite growth is probably
a consequence of formation in the P-rich environment,

7.3. The status of horákite and the related
minerals273

Jakub Plášil, Anthony R. Kampf, Jiří Sejkora, Jiří Čejka, Radek Škoda, Jaromír Tvrdý

Fig. 6 Graphical representation ofhorákite uranyl anion topology, which
consists of pentagons, squares and triangles. The large irregular green pentagons,
as well as the squares, are not occupied
in horákite.

structures (containing clusters of chains of U6+ polyhedra). The Bi uranyl–arsenate walpurgite has a higher
Bi : U proportion (4 : 1) than horákite (7 : 4).

7.4. Structural and chemical complexity andremarks on horákite origin
Horákite and related uranyl phosphates and arsenates arelisted in Tab. 7 along with their structural and chemical complexities. Horákite, with 685 bits/cell, qualifies
as a complex mineral in the classification scheme of
Krivovichev (2013). It is similar in complexity to phosphuranylite (766 bits/cell), which was found in the direct
association with horákite, and kamitugaite (536 bits/cell), which has a complicated sheet similar to that inphosphuranylite. Their high chemical complexities (> 100
bits/fu; see Plášil 2018) may relate to the fact that they
are all late-stage alteration phases with rather high Me
proportions or the presence of multiple chemical elements(derived during the weathering process).
Unambiguously, the most common Bi–U–O–H mineral
not only in Jáchymov, but in general, is walpurgite, asimple phase both structurally and chemically. Hügelite,with a very high complexity of 1950 bits/cell, is the only
phase in the list with complexity higher than horákite,
phosphuranylite and kamitugaite (chemical complexity:274

27–65 bits/fu, structural complexity: ~184 bits/cell).
Consequently, hügelite is a very rare mineral in nature.
On the studied specimen, horákite and phosphuranylite
occur in a close association; this may, in part, relate totheir very similar structural complexities. We can only
speculate that horákite formed contemporaneously withphosphuranylite.
Acknowledgements. We thank Pavel Škácha (Mining Museum Příbram, Czech Republic) for microphotography of
horákite. The paper benefits from the thorough reviewsof Travis Olds, František Veselovský and Frédéric Hatert.
This research was supported by the project of GAČR No.
17-09161S to J.P. and J.S.
Electronic supplementary material. Supplementary crystallographic data for this paper are available online at the
Journal web site (http://dx.doi.org/10.3190/jgeosci.267).

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